Frequency changing network



Patented Sept. 24, 1946 FREQUENCY CHANGING NETWORK Charles Malton Le Grice Eyre and John Owen England, assignors, by mesne assignments, to International Standard Electric Corporation, New York, N. Y., a cor- Gilderdale,

poration of Delaware London,

Application January 12, 1944, Serial No. 517,912 In Great Britain February 1, 1943 Claims.

The present invention relates to electric thermionic valve modulating arrangements, and is of particular application to superheterodyne receivers.

In superheterodyne receivers it is sometimes desirable to couple the frequency changing stage aperiodically to its load, and in particular to couple it aperiodically to a load of relatively low impedance. For example, it may be desired to construct a receiver in which the intermediate frequency is variable over a certain frequency band, the frequency of the beating oscillator being fixed, and the output of the frequency changer being delivered over a transmission line to a tunable intermediatefrequency amplifier. In such a case it will probably be very inconvenient to have to tune the output circuit of the frequenc changer in conjunction with the tuning of the intermediate frequency amplifier, since some kind of remotely controlled ganging arrangement would be needed. Moreover, proper impedance matching between the output circuit of the frequency changer and the transmission line practically necessitates that the output circuit impedance should be a pure resistance over the frequency range concerned, and this makes a semi-aperiodic arrangement impracticable.

Inasmuch as the load presented to the frequency changer by the transmission line and the input circuits of the intermediate frequency amplifiers may be very low for the form of receiver mentioned above, conventional frequency changing arrangements, with aperiodic coupling, would be very inefiicient.

The principal object of the invention is to provide a simple modulating arrangement which may be efficiently coupled and matched to an aperiodic load of low impedance.

According to the invention, there is provided an electrical modulating or demodulating arrangement comprising a thermionic valve having an impedance connected in series with the cathode in both the input and output circuits of the valve for the purpose of providing negative feedback between such circuits, means for simultaneously applying a signal wave and a carrier Wave to the input circuit of the valve, means for biassing the valve in such manner that the applied waves cause the anode current to vary over a substantially curved portion of the valve characteristic curve, and output means connected to the said impedanc for extracting the modulation products from the valve.

The invention also provides an electrical frequency changer comprising a thermionic valve arranged in a cathode follower circuit with a resistance in series with the cathode, means for applying simultaneously to the control grid two series of input waves having different frequencies, means for biassing the valve in such manher that it operates as a rectifier, and low impedance output means connected to the resistance for extracting therefrom waves having a frequency equal to the difference of the input frequencies.

According to another aspect, there is provided a modulating or demodulating circuit comprising a thermionic valve having a cathode, anode, and control grid, an input circuit containing two sources of electrical variations connected between the control grid and ground, a feedback path between the anode and control grid circuits comprising an impedance element connected between the cathode and ground, a low impedance aperiodic load connected across the said element, and means for conditioning the valve in such manner as to produce in the said load a beat variation from the said sources of variations.

A practical embodiment will be described in order to illustrate the invention.

The figure of the accompanying drawing shows a schematic circuit diagram of the embodiment which comprises a frequency changing arrangement. This is based on an experimental circuit which gave satisfactory results, and it contains a number of details of an unessential character which could have been provided in several other ways. In this figure the condensers designated K1 or K2 are by-pass or blocking condensers of negligible impedance at the frequencies concerned.

The circuit comprises a modulating valve V, shown as a pentode, having a resistance R1 connecting the cathode is to earth. The anode a is connected at 4 to the positive terminal of the high tension source I-ITI through resistances R3 and R4, and the screen grid in is connected to the same source through resistances R5 and Rs. By-pass condensers K1 are shown connecting the junction points of these resistances to earth to form a well known decoupling arrangement. The suppressor grid 92 is connected to earth,

The radio frequency signals and the carrier waves are supplied simultaneously to the control grid as from terminals I and 2 respectively. The inductance coilsLz, L3 and adjustable condensers C2, C3 form a radio frequency band filter through which the radio frequency signal and carrier waves are applied to the control grid 9/3. The radio frequency signals are applied to the center point of coil L2. They are obtained through radio frequency input terminals I from the anode circuit of a valve (not shown) which derives high tension voltage from the source HT2 at terminal 5 through the coil L2. The inductance .0011 L1 is a coupling coil through which the carrier Waves are supplied from oscillator input termi nals 2 to the control grid 93. and shunting by-pass condenser K1 are details of the input filtering arrangements and do not affect the operation of the valve V. The method of applying the waves to the valve is immaterial as regards the invention, and any other suitable means could be used instead.

The valve V should preferably have a high The resistance R2 5. V t

mutual conductance, and is so biassed, and/or the level of the input signals is so chosen, that the anode current swings over a substantially curved portion of the valve characteristic, whereby the valve acts as a modulator or rectifier.

The resistance Ri' sas a low value, and the intermediate frequency signals are taken from the terminals of this resistance through a coaxial transmission line T connected at terminals 3 to amplifying equipment (not shown). A large blocking condenser K2 is provided, but may not benecessary in all cases. The resistance R1 is shunted by a small condenser C1 for the purpose of by -passing the radio frequency waves.

It will be seen that as regards the intermediate frequency the arrangement is similar to the 'well known cathode follower amplifying circuit,

but the difference lies in the fact that the valve is operated so that it acts as a rectifier. The resistance R1 provides negative feedback at the intermediate frequency and there are thus obtained the wellknown advantages resulting from negative feedback. In particular, the resistance R1 'may be chosen so that the' low impedance of the co-axial line or other output circuit is properly matched. In this way an aperiodic coupling arrangement of maximum efficiency is obtained in a very simple way by which the intermediate frequency signals'may be directly extracted from the modulating valve without the use of any transformers or tuned circuits.

It'will be evident that many of the details shown in the figure are unessential, and any other arrangements performing the same functions could be used instead. The important points, however, are

(1) The resistance R1 should preferably be chosen so that the output load impedance is matched at the intermediate frequency.

(2) The radio frequency signals and carrier waves should be applied simultaneously to the valve.

(3) The valve should be so biassed and/ or the signal level should be so adjusted that the anode current swings over a substantially curved portion of the valve characteristic, or, in other words, so that the valve acts as a rectifier.

It will be evident that these requirements can be met in a number of other ways. For example, the valve V is not essentially a pentode; a triode or'tetrode could have been used, or a valve with two control grids to which the radio frequency signals and carrier waves are separately supplied.

In a particular-case of the circuit of the accompanying drawing which gave good results, the radio frequency was in the range 30 to 40 megacycles, and the intermediate frequency was variable over the range 3 to 4 megacycles. The coaxial line T had an impedance of 70 ohms.

4 The following details of the relevant components are given:

Valve V, Mazda SP. 41, R. F. pentode:

-Resistance R1 ohms 200 .jResistance R3 do 2,200

Resistance R4 d0 4,700

Resistance R5 do 10,000

Resistance Rs do 20,000 Condenser C1 micro-microfarads;

Condenser K1 rnicrofarad 0.002

, Condenser Kz do 0.1

I 7 Voltage HTl volts 220' 1 Under 'theseiconditions the cathode current was about"7;5 milliamps, so that the control grid was biassed negatively to the cathode by about 1.5 volts. The peak signal voltage applied to the control grid was arranged to be about 2 volts.

It should be clearly understood that these particulars are given for illustration only and in other circumstances quite different components and values might be used. The same principles are applicable whatever the ranges of the signal and intermediate frequencies may be; for example, the intermediatefrequency may be represented by the finally demodulated signals, which may be of audio frequency. The value would then be actingas a final demodulating valve. Thus the arrangement could be coupled efficiently and directly to a low impedance device such as a moving coil loud speaker. Similarly if the radio frequency output signals were replaced by the audio frequency output of a microphone, for example, the valve'would be acting as an initial modulator.

What is claimed is:

1. An electrical device for securing changes ofmodulation" by the algebraicaddition of waves of differing frequencies; including a thermionic valve having electrodes including'a cathode, input and output circuits therefor, an impedance connected in series with said cathode and inserted in both the input and output circuits of said valve'forming a negative feedback path between pedance for extractingthe modulation products from said valve. 1

2. An electrical frequency changer including 'a thermionic valve connected in a cathode follower circuit and having, electrodes including a control grid anda'cathode, a resistance'connected in series with the cathode of; said valve, means for applying simultaneously tosaid control grid two series of input waves having differing frequencies, means forbiassing said valve with potentialsso chosen that said valve operates as a rectifier, and low impedance output means connected'to at least a 'portion of said resistance for extracting therefrom'waves having-a frequency equal to thedifference of said input waves of differing frequenmes.

3. A modulation'changing circuit. including a thermionic valve: having a cathode, anode and control grid, an input circuit for said valve, means for simultaneously; supplying said input' circuit with two series of electrical variations of differing frequencies, means for connecting said input cir-' trol grid circuits and comprising an impedance element connected between said cathode and ground, a low impedance aperiodic 1oad connected across at least part of said impedance element, and means for conditioning said valve with potentials s0 chosen as to produce in said load a beat variation derived from said sources of electrical variations.

4. A device according to claim 2, in which said thermionic valve is of the type having a screen 10 grid intermediate between the control grid and the anode thereof, and means for applying a suitable potential to said screen grid.

5. A d vice according to claim 3, in which said aperiodic load connected to said impedance element includes a co-axial transmission line, whereby all load tuning can be effected at the farther end of said transmission line.

CHARLES MALTON LE GRICE EYRE. JOHN OWEN GILDERDALE. 

